1. Field of the invention
The present invention relates to a power transformer combined with balance windings and application circuits thereof and, more particularly, to a power transformer and application circuits thereof, which provide stable induction voltages and balance load currents to CCFLs to drive the CCFLs so as to enhance the luminous efficiency and uniformity of the CCFLs.
2. Description of Related Art
An ordinary LCD panel utilizes cold cathode fluorescent lamps (CCFLs) as its backlight. Generally speaking, an inverter circuit is used to drive the CCFLs. In order to meet the requirement of a high voltage output, however, a high frequency transformer is adopted to drive the CCFLs.
As shown in FIG. 1, a prior art high frequency transformer is composed of a bobbin 1a, a first ferrite core 1b and a second ferrite core 1c. A primary winding region 1d and a secondary winding region 1e are formed on the bobbin 1a. 
As shown in FIG. 1, a secondary winding 1g is wound around a plurality of separation grooves 1h projectively disposed at the side edge of the bobbin 1a to form the secondary winding region 1e, and a primary winding 1f is wound around other regions of the bobbin 1a to form the primary winding region 1d. The prior art high frequency transformer can thus be connected to a power source and a load. With the gradual increase of the size of LCD panel, however, the length and quantity of CCFLs increase therewith. Therefore, the required drive voltage of the transformer also increases therewith. Based on the voltage division principle of the separation grooves 1h, it is necessary to provide a corresponding number of separation grooves 1h according to the magnitude of the voltage used so as to enhance the reliability of product.
As shown in FIG. 2, because the impedances of the CCFLs 2a and 2b show negative temperature dependence, their impedances drop due to an increase in temperatures after conduction. According to the ideal usage state of load, a secondary winding 1g is often correspondingly in shunt with the CCFLs 2a and 2b in consideration of cost. The impedances of the CCFLs 2a and 2b, however, are different. Therefore, a current division effect easily arises at the lamp ends of the CCFLs 2a and 2b to cause uneven currents of the CCFLs 2a and 2b, hence resulting in a too large a difference in their brightness.
As shown in FIGS. 3A and 3B, in order to conquer the above problem of uneven currents of the CCFLs 2a and 2b, a matched balance winding 3 is generally added to accomplish mutual current induction of the CCFLs 2a and 2b so as to acquire more balanced currents, thereby effectively improving the brightness. However, adding a matched balance winding 3 not only increases the cost, but also limits the space usage of a circuit board, hence in practice it is not a good design.
Besides, as shown in FIG. 3, if high voltage capacitors C1 and C2 in FIG. 2 are replaced with inductive elements L1 and L2, the object of reducing the current division effect can also be achieved. When a prior art power transformer is used to drive several lamps, the magnitude of current used multiplies with the number of lamps. The diameter of wire used at the secondary side thus increases to make the space of the winding region insufficient. There is therefore no choice but to enlarge the winding region. But adopting a larger power transformer raises costs. Therefore, in consideration of the characteristics of CCFLs and the cost, balance windings are combined in a power transformer in the present invention to improve the above drawbacks of prior art drive circuits.